[selectors-4] Add :is() example in specificity section, and add it in Overview section too

Author: LeaVerou

selectors-4/Overview.bs

@@ -131,6 +131,12 @@ Selectors Overview</h2>
 						and/or <a>compound selector</a> <var>s2</var>
 				<td>[[#matches]]
 				<td>4
+			<tr>
+				<td><code>E:is(<var>s1</var>, <var>s2</var>)</code>
+				<td>an E element that matches <a>compound selector</a> <var>s1</var>
+						and/or <a>compound selector</a> <var>s2</var> but contributes no specificity.
+				<td>[[#is]]
+				<td>4
 			<tr>
 				<td><code>E:has(<var>rs1</var>, <var>rs2</var>)</code>
 				<td>an E element,
@@ -3127,6 +3133,9 @@ Calculating a selector's specificity</h2>
 				'':matches(em, #foo)'' has
 				a specificity of (0,0,1)--like a tag selector--when matched against <code>&lt;em></code>,
 				and a specificity of (1,0,0)--like an ID selector--when matched against  <code>&lt;em id=foo></code>.
+			<li>
+				''div:is(em, #foo#bar#baz)'' has
+				a specificity of (0,0,1): only the ''div'' contributes to selector specificity.
 			<li>
 				'':nth-child(even of li, .item)'' has
 				a specificity of (0,1,1)--like a tag selector plus a pseudo-class--when matched against <code>&lt;li></code>,